I was initially going to call this category Shoot 'em ups (as that's what I would call them), but because I want it to include light-gun games, and I didn't want to get it in the neck like I did on the beat 'em up category.

As ever, the definition is up to you but here is what Wikipedia has to say about them:

Shooter games are a subgenre of action game, which often test the player's speed and reaction time. It includes many subgenres that have the commonality of focusing on the actions of the avatar using some sort of weapon. Usually this weapon is a gun, or some other long-range weapon. A common resource found in many shooter games is ammunition. Most commonly, the purpose of a shooter game is to shoot opponents and proceed through missions without the player character being killed or dying. A shooting game is a genre of video game where the player has limited spatial control of his or her character, and the focus is almost entirely on the defeat of the character's enemies using long-range weaponry.

I'm just trying to cover as many games as possible, otherwise they won't get a category. You might classify Gears of War as a shooting game, you might classify it as an action game - it's up to you, but I don't want to be restrictive because genres are so subjective to people anyway.

For me, the focus in Splatoon is on shooting, which makes it a shooting game in my opinion. Gears of War there is a bit more focus on story, etc which for me makes it an action game.